Cells proliferate, migrate, and differentiate in spacially and temporally specific manners during the course of normal embryonic development. How these processes are regulated, and how they regulate each other, is not understood. The skin, and the epidermis in particular, is an excellent model for the study of some of the fundamental processes directing normal development. The aim of this proposal is to characterize the expression of developmentally regulated epidermal genes, to study changes in cell proliferation and the cell surface during the crucial stage of epidermal morphogenesis known as stratification, and to perturb epidermal proliferation and stratification in order to establish the affects that these processes have upon the expression of differentiation-specific genes. To understand the regulation of epidermal proliferation, and the relationship of this process to epidermal differentiation, the expression of certain cellular oncogenes will be followed. The study will utilize both human and mouse skin to take advantage of the strengths offered by each system and to establish general aspects of epidermal regulation common to both species. The human embryonic epidermis is well characterized, and a system for the study of stratification of human embryonic skin in vitro has been established. The expression of oncogenes and cell surface receptors for the peanut agglutinin lectin will be studied in developing human epidermis during the period of epidermal stratification. In addition, the expression of differentiation-specific keratins will be studied in relation to cell proliferation in stratifying organ cultures of human fetal skin by double-labelling with antibodies against BrdUrd and with monospecific anti-keratin antibodies. Stratification of human skin in organ culture will be perturbed with drugs and/or with anti-PNA receptor antibodies in order to determine the influence of stratification on expression of markers of epidermal differentiation. On the other hand, the developing mouse offers the opportunity to study a system that is easy to manipulate and in which several mutations affecting epidermal development have been identified. Cell proliferation, and expression of keratins and certain cellular oncogenes, will be studied during normal development of the mouse with particular reference to epidermal stratification, and in two mutations of mice causing abnormal epidermal stratification and differentiation: pupoid fetus (pf) and repeated epilation (Er). In summary, this study seeks to understand the relationship between the processes of stratification, proliferation, and differentiation during the course of embryonic development of mouse and man so that general mechanisms regulating epidermal development may be established.